U.S. patent application number 11/059505 was filed with the patent office on 2005-07-07 for developing liquid coating device, developing device including the same and image forming apparatus including the developing device.
Invention is credited to Takeuchi, Noriyasu, Yoshino, Mie.
Application Number | 20050147431 11/059505 |
Document ID | / |
Family ID | 30002184 |
Filed Date | 2005-07-07 |
United States Patent
Application |
20050147431 |
Kind Code |
A1 |
Takeuchi, Noriyasu ; et
al. |
July 7, 2005 |
Developing liquid coating device, developing device including the
same and image forming apparatus including the developing
device
Abstract
A device for coating a developing liquid included in an
developing device of the present invention includes a coating
member rotatable with the developing liquid deposited thereon in a
preselected amount. An, intermediate member has a surface
contacting the surface of the coating member and is movable at the
same speed and in the same direction as the surface of the coating
member. Also, the surface of the intermediate member contacts the
surface of a developing roller to be coated with the developing
liquid and is movable in the opposite direction to the surface of
the developing roller.
Inventors: |
Takeuchi, Noriyasu;
(Kawasaki-shi, JP) ; Yoshino, Mie; (Kawasaki-shi,
JP) |
Correspondence
Address: |
OBLON, SPIVAK, MCCLELLAND, MAIER & NEUSTADT, P.C.
1940 DUKE STREET
ALEXANDRIA
VA
22314
US
|
Family ID: |
30002184 |
Appl. No.: |
11/059505 |
Filed: |
February 17, 2005 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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11059505 |
Feb 17, 2005 |
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10299698 |
Nov 20, 2002 |
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6868246 |
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Current U.S.
Class: |
399/237 |
Current CPC
Class: |
G03G 15/101 20130101;
G03G 15/104 20130101 |
Class at
Publication: |
399/237 |
International
Class: |
G03G 015/10 |
Foreign Application Data
Date |
Code |
Application Number |
Nov 20, 2001 |
JP |
2001-354109 (JP) |
Jul 5, 2002 |
JP |
2002-196763 (JP) |
Claims
1-54. (canceled)
55. A method of coating a developing liquid, which is deposited on
a surface of a rotatable coating member in a preselected amount, on
a surface of a subject member to be coated, said method comprising:
preparing an intermediate member having a surface contacting the
surface of said coating member and movable at a same speed and in a
same direction as said surface of said coating member, and
contacting the surface of said subject member and movable in an
opposite direction to said surface of said subject member; coating
the developing liquid deposited on the surface of said coating
member on the surface of said intermediate member; coating the
developing liquid deposited on the surface of said intermediate
member on the surface of said subject member; and rotating the
intermediate member in a direction opposite to the direction of the
surface of the coating member.
56. A device for coating a developing liquid, comprising: a coating
member rotatable with the developing liquid deposited on a surface
thereof in a preselected amount; an intermediate member having a
surface contacting a surface of said coating member and movable at
a same speed and in a same direction as said surface of said
coating member, and contacting a surface of a subject member to be
coated and movable in an opposite direction to said surface of said
subject member; and a drive mechanism configured to rotate the
intermediate member in a direction opposite to the direction of the
surface of the coating member.
57. In a developing device for developing a latent image formed on
a surface of an image carrier with a developing liquid on a surface
of a developer carrier by a coating device, said coating device
comprising: a coating member rotatable with the developing liquid
deposited on a surface thereof in a preselected amount; an
intermediate member having a surface contacting the surface of said
coating member and movable at a same speed and in a same direction
as said surface of said coating member, and contacting a surface of
said developer carrier and movable in an opposite direction to said
surface of said developer carrier; and a drive mechanism configured
to rotate the intermediate member in a direction opposite to the
direction of the surface of the coating member.
58. An image forming apparatus comprising: an image carrier; image
forming means for forming a latent image on a surface of said image
carrier; and a developing device for developing the latent image
with a developing liquid; said developing device comprising: a
coating member rotatable with the developing liquid deposited on a
surface thereof in a preselected amount; an intermediate member
having a surface contacting the surface of said coating member and
movable at a same speed and in a same direction as said surface of
said coating member, and contacting a surface of a developer
carrier to be coated and movable in an opposite direction to said
surface of said developer carrier; and a drive mechanism configured
to rotate the intermediate member in a direction opposite to the
direction of the surface of the coating member.
59. A device for coating a developing liquid, comprising: a coating
member rotatable with the developing liquid deposited thereon in a
preselected amount; an intermediate member interposed between said
coating member and a subject member to be coated with the
developing liquid, said intermediate member having a surface
contacting a surface of said coating member and movable at a same
speed and in a same direction as said surface of said coating
member, and contacting a surface of said subject member and movable
in an opposite direction to said surface of said subject member;
and a drive mechanism configured to rotate the intermediate member
in a direction opposite to the direction of the surface of the
coating member.
60. A developing device for developing a latent image formed on an
mage carrier with a developing liquid deposited on a surface of a
developer carrier in a thin layer, said developing device
comprising: a coating device comprising a coating member rotatable
with the developing liquid deposited thereon in a preselected
amount, an intermediate member interposed between said coating
member and said developer carrier to be coated with the developing
liquid, said intermediate member having a surface contacting a
surface of said coating member and movable at a same speed and in a
same direction as said surface of said coating member, and
contacting the surface of said developer carrier and movable in an
opposite direction to said surface of said developer carrier and a
drive mechanism configured to rotate the intermediate member in a
direction opposite to the direction of the surface of the coating
member; and a moving mechanism for selectively moving said
developer carrier into or out of contact with said image
carrier.
61. A developing device for developing a latent image formed on an
image carrier with a developing liquid deposited on a surface of a
developer carrier in a thin layer, said developing device
comprising: a coating device comprising a coating member rotatable
with the developing liquid deposited thereon in, a preselected
amount, an intermediate member interposed between said coating
member and said developer carrier to be coated with the developing
liquid, said intermediate member having a surface contacting a
surface of said coating member and movable at a same speed and in a
same direction as said surface of said coating member, and
contacting the surface of said developer carrier and movable in an
opposite direction to said surface of said developer carrier, and a
drive mechanism configured to rotate the intermediate member in a
direction opposite to the surface of the coating member; and a
container hermetically closed except for an inlet port for feeding
the developing liquid, an outlet port for discharging said
developing liquid and an opening via which said developing carrier
contacts said image carrier, said container accommodating said
coating device and said developer carrier.
62. A device for coating a developing liquid, comprising: a coating
member rotable with the developing liquid deposited thereon in a
preselected amount; an intermediate member interposed between said
coating member and a subject member to be coated with the
developing liquid, said intermediate member having a surface
contacting a surface of said coating member and movable at a same
speed and in a same direction as said surface of said coating
member, and contacting a surface of said subject member and movable
in an opposite direction to said surface of said subject member; a
moving mechanism for selectively moving said intermediate member
into or out of contact with said coating member; and a drive
mechanism configured to rotate the intermediate member in a
direction opposite to the surface of the coating member.
63. An image forming apparatus comprising: an image carrier for
forming a latent image thereon; a developing device for developing
the latent image with a developing liquid deposited on a surface of
a developer carrier, which is movable in contact with a surface of
said image carrier; and a coating device for coating the developing
liquid on the surface of said developer carrier, said coating
device comprising a coating member rotatable with the developing
liquid deposited thereon in a preselected amount, an intermediate
member interposed between said coating member and said developer
carrier to be coated with said developing liquid, said intermediate
member having a surface contacting a surface of said coating member
and movable at a same speed and in a same direction as said surface
of said coating member, and contacting the surface of said
developer carrier and movable in an opposite direction to said
surface of said developer carrier, a moving mechanism for
selectively moving said intermediate member into or out of contact
with said coating member, and a drive mechanism configured to
rotate the intermediate member in a direction opposite to the
surface of the coating member.
64. A device for coating a developing liquid on a surface of a
developer carrier, said coating device comprising: a coating member
rotatable with the developing liquid deposited thereon in a
preselected amount; an intermediate member having a surface
contacting a surface of said coating member and movable at a same
speed and in a same direction as said surface of said coating
member, and contacting the surface of said developer carrier and
movable in an opposite direction to said surface of said developer
carrier, said intermediate member having surface roughness of 3
.mu.m or above in terms of ten-point mean roughness Rz: and a drive
mechanism configured to rotate the intermediate member in a
direction opposite to the surface of coating member.
65. A device for coating a developing liquid on a surface of a
developer carrier, said coating device comprising: a coating member
rotatable with the developing liquid deposited thereon in a
preselected amount; an intermediate member having a surface
contacting a surface of said coating member and movable at a same
speed and in a same direction as said surface of said coating
member, and contacting the surface of said developer carrier and
movable in an opposite direction to said surface of said developer
carrier, and developer removing means contacting a position of the
surface of said intermediate member between said developer carrier
and said coating member, which are respectively positioned
downstream and upstream in a direction of movement of the surface
of said intermediate member when the developing liquid is to be
coated on the surface of said developer carrier, for removing said
developing liquid deposited on the surface fo said intermediate
member; and a drive mechanism configured to rotate the intermediate
member in a direction opposition to the surface of the coating
member, said intermediate member having surface roughness of 15
.mu.m or below in terms of ten-point mean surface roughness Rz.
66. An image forming apparatus comprising: an image carrier for
foming a latent image thereon; latent image forming means for
forming the latent image on surface of said image carrier; and
developing means for developing the latent image; said developing
means comprising: a coating member rotatable with a developing
liquid deposited thereon in a preselected amount; an intermediate
member having a surface contacting a surface of said coating member
and movable at a same speed and in a same direction as said surface
of said coating member, and contacting a surface of a developer
carrier to be coated with the developing liquid and movable in an
opposite direction to said surface of said developer carrier, said
intermediate member having surface roughness of 3 .mu.m or above in
terms of ten-point mean roughness Rz; and a drive mechanism
configured to rotate the intermediate member in a direction
opposite to the surface of the coating member.
67. An image forming apparatus comprising: an image carrier for
forming a latent image thereon; latent image forming means for
forming the latent image on a surface of said image carrier; and
developing means for developing the latent image; said developing
means comprising: a coating member rotatable with a developing
liquid deposited thereon in a preselected amount; an intermediate
member having a surface contacting a surface of said coating member
and movable at a same speed and in a same direction as said surface
of said coating member, and contacting a surface of a developer
carrier to be coated with the developing liquid and movable in an
opposite direction to said surface of said developer carrier; and
developing removing means contacting a position of the surface of
said intermediate member between said developer carrier and said
coating member, which are respectively positioned downstream and
upstream in a direction of movement of the surface of said
intermediate member when the developing liquid is to be coated on
the surface of said developer carrier, for removing said developing
liquid deposited on the surface of said intermediate member; and a
drive mechanism configured to rotate the intermediate member in a
direction opposite to the surface of the coating member, said
intermediate member having surface roughness of 15 .mu.m or below
in terms of ten-point mean surface roughness Rz.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] The present invention relates to a device for coating a
subject member with a developing liquid deposited on a coating
member in a preselected amount, a developing device for developing
a latent image formed on an image carrier with the developing
liquid deposited on a developer carrier, and a copier, facsimile
apparatus, printer or similar image forming apparatus including the
developing device.
[0003] 2. Description of the Background Art
[0004] An image forming apparatus of the type developing a latent
image formed on an image carrier with a highly viscous and dense
developing liquid is disclosed in, e.g., Japanese Patent Laid-Open
Publication Nos. 7-152254, 7-209922, and 7-219355. In this type of
image forming apparatus, an optical writing unit scans the surface
of an image carrier or photoconductive element uniformly charged by
a charger in accordance with image data, thereby forming a latent
image on the image carrier. A developing device develops the latent
image with a developing liquid to thereby produce a corresponding
toner image.
[0005] The developing device includes a coating device configured
to uniformly coat the developing liquid stored in a reservoir on a
developer carrier in a thin layer. The developer carrier is
implemented as a developing roller or a developing belt by way of
example and adjoins the surface of the image carrier. The
developing liquid coated on the developer carrier contacts the
surface of the image carrier in a developing zone where the
developer carrier and image carrier are positioned close to each
other. As a result, toner contained in the thin layer of the
developing liquid develops the latent image formed on the image
carrier for thereby producing a corresponding toner image. A blade
or similar cleaning member removes the developing liquid left on
the developer carrier after development and causes it to be
returned to the reservoir.
[0006] Subsequently, the toner image is transferred from the image
carrier to a sheet, OHP (OverHead Projector) film or similar
recording medium and then fixed by a fixing device. A drum cleaner
removes residual toner left on the image carrier after the image
transfer.
[0007] The developing liquid consists of an insulative carrier
liquid and solid toner dispersed in the carrier liquid and made up
of resin and pigment. For example, the developing liquid has
viscosity as high as 50 mPa.S to 10,000 mPa.S and consists of a
solvent implemented by an insulative liquid of dimethylpolysiloxane
oil and toner grains densely dispersed in the liquid. When the
developing liquid contacts the surface of the image carrier,
charged toner grains are electrostatically transferred from the
developing liquid to the image carrier to thereby develop the
latent image.
[0008] The amount of toner to be migrated through the developing
liquid and deposited on the latent image is inversely proportional
to the distance over which the, toner moves in the developing zone.
Stated another way, the shorter the distance of movement of the
toner in the developing zone, the higher the developing efficiency
available for the latent image. To reduce this distance, the
developing liquid should preferably form a layer as thin as the
order of microns on the developer carrier and contact the image
carrier. This is particularly true when the viscosity of the
developing liquid is as high as 50 mPa.S to 10,000 mPa.S.
[0009] When the thin layer of the developing liquid develops the
latent image, the density of the resulting toner image is
determined by the thickness of the layer. In this respect, the
thinness of the developer layer formed on the developer carrier is
the key to desirable image density. In light of this, use is made
of, e.g., a coating device including a coating member for coating
the developing liquid on the developer carrier. The coating member
may be implemented as a coating roller carved with cells in a
uniform pattern (so-called photogravure roller), as taught in
Japanese Patent Laid-Open Publication No. 11-265122 by way of
example. After the developing liquid has been deposited on such a
coating roller, a doctor blade or metering member held in contact
with the coating roller removes excessive part of the developing
liquid, thereby metering the developing roller deposited on the
coating roller. The metered developing liquid is directly coated
on, or transferred to, the developer carrier, forming a uniform
thin layer on the developer carrier.
[0010] In the conventional coating device described above, the
developing liquid is directly transferred from the coating roller
to the developer carrier, as stated above. Therefore, the problem
with the developing device, in which the coating roller and
developer carrier rotate in contact with each other, is that the
coating roller is likely to shave off the developer carrier with
its uniform cell pattern, accelerating exhaustion of and damage to
the developer carrier.
[0011] It follows that the developer carrier included in the
developing device of the type described must satisfy the following
conditions (1) through (5).
[0012] (1) The hardness of the developer carrier is low enough to
form a preselected nip for development between the developer
carrier and the image carrier.
[0013] (2) At least the surface of the developer carrier is
conductive and capable of being applied with a bias.
[0014] (3) At least the surface of the developer carrier has
mechanical strength great enough to resist wear ascribable to
friction, which acts between the developer carrier and the coating
roller.
[0015] (4) At least the surface of the developer carrier has
mechanical strength great enough to resist wear ascribable to
friction, which acts between the developer carrier and the cleaning
blade.
[0016] (5) The surface of the developer carrier is smooth enough to
uniformly coat the developing liquid on the image carrier.
[0017] The above conditions (3) and (4) relate to the durability of
the developer carrier and therefore determines the life of the
same. In the conventional coating device in which the photogravure
roller or similar coating roller with a carved surface rotates in
direct contact with, e.g., a developing roller, the life of the
developing roller corresponds to only 50,000 prints or so even if
it is covered with a conductive PFA tube, as determined by a
continuous image forming test. Further, the material applicable to
the developer carrier is limited due to the above severe conditions
required of the developer carrier. It is therefore difficult to
provide the developer carrier with durability that satisfies the
conditions (1) through (5).
[0018] To solve the above problem, there has been proposed. a
coating device including an intermediate roller (or belt)
interposed between a coating roller (or belt) or coating member and
a developing roller (or belt) or subject member to be coated. In
this coating device, a developing liquid is transferred from the
coating roller to the developing roller via the intermediate
roller, i.e., the coating roller does not contact the developing
roller. The developing roller is therefore free from wear and
damage ascribable to the contact thereof with the coating roller
and achieves a longer life.
[0019] In a developing system of the type developing a latent image
formed on an image carrier with a developing liquid coated on a
developing roller or developer carrier, as stated above, whether or
not the liquid can stably form a thin layer on the developing
roller is the key to high image quality. However, when the
intermediate roller or intermediate member is formed of an
insulative material, the developing liquid cannot form a uniform
thin layer on the developer carrier due to the frictional charging
of the surface of the intermediate roller.
[0020] In the developing device with the coating device described
above, a sufficient nip for development should preferably be formed
between the developing roller and the image carrier in order to
stabilize image quality. For this purpose, the developer carrier
may include an elastic layer of low hardness such that the
developer carrier deforms when pressed against the image carrier,
thereby forming the desired nip. However, the developer carrier
suffers from permanent set if left in pressing contact with the
image carrier when the developing device is not operated. The
permanent set causes the amount of the developing liquid to deposit
on the image carrier and therefore image density to vary.
[0021] Still another problem with the developing device of the type
described is that when impurities are introduced in the developing
liquid stored in the coating device, stripes extend from the
impurities over the circumference of the developer carrier,
lowering image quality. Moreover, if such impurities are harder
than any one of the coating member, intermediate member and
developer carrier, then the former scratches the surface of the
latter and thereby reduces the life or the same.
SUMMARY OF THE INVENTION
[0022] It is an object of the present invention to provide a
developing liquid coating device capable of extending the life of a
developer carrier, a developing device including the same, and an
image forming apparatus including the developing device.
[0023] It is another object of the present invention to provide a
developing liquid coating device capable of causing a developing
liquid to form a uniform thin layer on a developer carrier, a
developing device including the same, and an image forming
apparatus including the developing device.
[0024] It is a further object of the present invention to provide a
developing liquid coating device capable of uniformly coating a
developing liquid on a developer carrier until the end of the
developer carrier, a developing device including the same, and an
image forming apparatus including the developing device.
[0025] A device for coating a developing liquid of the present
invention includes a coating member rotatable with the developing
liquid deposited thereon in a preselected amount. An intermediate
member has a surface contacting the surface of the coating member
and movable at the same speed and in the same direction as the
surface of the coating member. Also, the surface of the
intermediate member contacts the surface of a subject member to be
coated and movable in the opposite direction to the surface of the
subject member.
BRIEF DESCRIPTION OF THE DRAWINGS
[0026] The above and other objects, features and advantages of the
present invention will become more apparent from the following
detailed description taken with the accompanying drawings in
which:
[0027] FIG. 1 is a view showing major part of a first embodiment of
the image forming apparatus in accordance with the present
invention;
[0028] FIG. 2 is a section showing an intermediate roller included
in a coating device that forms part of a developing device included
in the illustrative embodiment;
[0029] FIG. 3 is a section showing a developing roller and a sweep
roller also included in the developing device;
[0030] FIG. 4 shows another specific configuration of the
illustrative embodiment;
[0031] FIG. 5 is a front view showing a specific configuration of a
coating roller included in the coating device and implemented as a
photogravure roller;
[0032] FIGS. 6A through 6C are enlarged isometric views each
showing a particular cell pattern formed on the coating roller;
[0033] FIG. 7 is a graph showing a relation between the strength of
an electric field for development and a development ratio;
[0034] FIG. 8 is a graph showing a relation between a speed ratio
between the developing roller and the intermediate roller and the
amount of a developing liquid deposited on the developing
roller;
[0035] FIG. 9 shows a specific configuration of a drive mechanism
capable of driving the coating roller and intermediate roller with
a single drive source while maintaining their peripheral speeds
equal to each other;
[0036] FIG. 10 shows a specific configuration of a drive mechanism
confiqured to drive the intermediate roller with a single drive
source while causing the coating roller to be rotated by the
intermediate roller;
[0037] FIG. 11 shows a specific configuration of a drive roller
configured to drive the coating roller and intermediate roller with
a single drive source while obviating the slip of the intermediate
roller;
[0038] FIG. 12 shows the position of the intermediate roller with
respect to the developing roller and coating roller;
[0039] FIG. 13 shows a specific configuration of the coating device
configured to maintain the density of a toner image constant by
using a density sensor;
[0040] FIG. 14 shows a relation in width between the coating
roller, the intermediate roller, the developing roller and a
photoconductive drum;
[0041] FIGS. 15A and 15B show a relation in width between the
coating roller and the intermediate roller;
[0042] FIG. 16 shows a relation in width between the coating
roller, the intermediate roller, the developing roller and the
drum;
[0043] FIGS. 17A and 17B show a relation in width between the
coating roller, the intermediate roller, the developing roller and
a cleaning blade;
[0044] FIG. 18 shows a relation in width between the intermediate
roller, the developing roller and the drum;
[0045] FIG. 19 shows a relation in width between the intermediate
roller, developing roller and rum and the cells of the coating
roller;
[0046] FIG. 20 is a view showing major part of a second embodiment
of the image forming apparatus in accordance with the present
invention;
[0047] FIG. 21 is a section showing an intermediate roller included
in a coating device that forms part of a developing device included
in the second embodiment;
[0048] FIG. 22 shows a relating between the surface resistance of
the intermediate roller of FIG. 21 and the coating condition;
[0049] FIG. 23 shows a developing roller included in the developing
device in a position released from a photoconductive drum;
[0050] FIG. 24 shows a moving mechanism for selectively moving the
developing roller into or out of contact with the drum;
[0051] FIG. 25 shows the developing roller released from the drum
by the moving means;
[0052] FIG. 26 demonstrates the flow of a developing liquid in the
coating device;
[0053] FIG. 27 is a fragmentary view showing a mechanism for
driving a shutter included in the coating device;
[0054] FIG. 28 shows a condition wherein the developing liquid
exists at a nip between the intermediate roller and the developing
roller;
[0055] FIG. 29 shows another moving mechanism for selectively
moving the developing roller into or out of contact with the
drum;
[0056] FIG. 30 shows the developing roller released from the drum
by the moving mechanism of FIG. 29;
[0057] FIG. 31 shows still another moving mechanism for selectively
moving the developing roller into or out of contact with the
drum;
[0058] FIG. 32 shows the developing roller released from the drum
by the moving mechanism of FIG. 31;
[0059] FIG. 33 shows major part of a third embodiment of the image
forming apparatus in accordance with the present invention;
[0060] FIG. 34 shows a relation between the surface roughness of an
intermediate roller included in a coating device, which forms part
of a developing device included in the illustrative embodiment, and
a roller life;
[0061] FIG. 35 is a graph showing a relation between the surface
roughness of the intermediate roller and the transfer ratio of a
developing liquid to a developing roller also included in the
developing device;
[0062] FIG. 36 is a view for describing two different conditions in
which a cleaning blade may contact the intermediate roller;
[0063] FIGS. 37A through 37C demonstrate the behavior of impurities
caught between the intermediate roller and the cleaning blade;
[0064] FIG. 38 is a graph showing set values available with a
stepping motor or drive source for driving the intermediate
roller;
[0065] FIGS. 39A and 39B demonstrate the behavior of a removing
member for removing the impurities; and
[0066] FIG. 40 is a graph showing a relation between the surface
roughness of the intermediate roller after coating and the surface
roughness of the same before coating.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0067] Preferred embodiments of the image forming apparatus in
accordance with the present invention will be described
hereinafter.
First Embodiment
[0068] Referring to FIG. 1 of the drawings, an image forming
apparatus embodying the present invention is shown and implemented
as an electrophotographic copier by way of example. As shown, the
copier includes an image carrier implemented as a photoconductive
drum 1. Arranged around the drum 1 are various image forming means
including a charger 2, an optical writing unit 3, a developing
device 4, an image transferring device 5, and a cleaning device 6.
The drum 1 may be formed of amorphous silicon (a-Si), organic
photoconductor (OPC) or similar material. The charger 2 may be
implemented as, e.g., a charge roller. The optical writing unit 3
may include an LED (Light Emitting Device) array or laser
optics.
[0069] The copier with the above construction forms an image by
reversal development, as will be described hereinafter. A motor or
similar drive means, not shown, causes the drum 1 to rotate at
constant speed in a direction indicated by an arrow in FIG. 1. The
charger 2 uniformly charges the surface of the drum 1, which is in
rotation, in the dark. The optical writing unit 3 scans the charged
surface of the drum 1 with a light beam in accordance with image
data, thereby forming a latent image representative of a document
image. When the latent image formed on the drum 1 is brought to a
developing zone A, the developing device 4 develops the latent
image with a developing liquid to thereby produce a corresponding
toner image.
[0070] A bias for image transfer is applied to an image transfer
roller 501 included in the image transferring device 5, which is
located at an image transferring zone B. The image transfer roller
501 applied with the bias transfers the toner image from the drum 1
to a belt or intermediate image transfer body 502, which forms part
of the image transferring device 5. This image transfer will be
referred to as primary image transfer. The toner image is then
transferred from the belt 502 to a sheet, OHP film or similar
recording medium at a secondary image transferring zone by
secondary image transferring means, although not shown
specifically. A fixing unit, not shown, fixes the toner image on
the sheet. Finally, the sheet with the fixed toner image is driven
out of the copier as a print.
[0071] A quenching lamp 7 dissipates potential left on the drum 1,
i.e., discharges the drum 1 after the primary image transfer. Also,
the cleaning device 6 removes the toner left on the drum 1 after
the primary image transfer. While the image transferring device 5
is implemented as a roller in the illustrative embodiment, it may
alternatively be implemented by corona discharge, adhesion transfer
or thermal transfer, if desired. The fixing unit may use thermal
transfer or fixation using a solvent, ultraviolet rays or pressure
by way of example.
[0072] In the illustrative embodiment, the developing liquid used
to develop a latent image is a highly viscous and dense developing
liquid different from a conventional low viscosity (about 1 cSt),
low density (about 1%) developing liquid using Isopar (trade name)
available from EXXON as a carrier liquid. The viscosity of the
developing liquid used in the illustrative embodiment should
preferably be between 50 mPa.S and 10,000 mPa.S while the density
or toner content should preferably be between 5% and 40%. As for
the carrier liquid, use may be made of, e.g., normal paraffin,
Isopar (trade name) available from EXXON, plant oil, mineral oil or
similar highly insulative oil. The developing liquid may be either
volatile or nonvolatile in accordance with the purpose. Toner
grains contained in the developing liquid may have any suitable
grain size between the order of submicrons and about 6 microns.
[0073] The developing device 4, which characterizes the
illustrative embodiment, includes a reservoir 401 storing a
developing liquid D. A developing roller 402 is rotatably disposed
in the reservoir 401 while a sweep roller 403 is rotatably disposed
in a casing 410. A coating device for coating the developing liquid
D on the developing roller, or subject member to be coated, 402
includes a coating roller 404, an intermediate roller or
intermediate member 405, and a screw 406. The surface of the
coating roller 404 is carved with a uniform pattern. The
intermediate roller 405 coats the developing liquid on the
developing roller 402 while the screw 406 conveys the developing
liquid D while agitating it. Cleaning, members 407 and 408 are held
in contact with the developing roller 402 and sweep roller 403,
respectively, and each is implemented as a blade or a roller formed
of metal or rubber.
[0074] In the coating device, the intermediate roller 405
bifunctions as a doctor roller for removing excess part of the
developing liquid D from the coating roller 404 to thereby regulate
the amount of the liquid D deposited on the roller 404. The
developing liquid D coated on the coating roller 404 is transferred
to the intermediate roller 405 in an amount metered by a doctor
blade or metering member 409.
[0075] As shown in FIG. 2, the intermediate roller 405 is made up
of a metallic core 405a, an elastic layer 405b formed on the core
405a, and a smooth layer 405c formed on the elastic layer 405b. The
elastic layer 405b may be formed of urethane rubber or similar
material that does not swell with or dissolve in the carrier liquid
or the developing liquid.
[0076] As shown in FIG. 3, the developing roller 402 is made up of
a metallic core 402a, an elastic layer 402b formed on the core
402a, and a conductive layer 402c formed on the elastic layer 402b.
Likewise, the sweep roller 403 is made up of a metallic core 403a,
an elastic layer 403b formed on the core 403a, and a conductive
layer 403c formed on the elastic layer 403b. The elastic layers
402b and 403b may be formed of urethane rubber. The elastic layers
402b and 403b should preferably have rubber hardness of 50.degree.
or less in terms of JIS (Japanese Industrial Standards)-A scale, so
that the developing roller 402 and sweep roller 403 can efficiently
form a nip for development and a nip for removal, respectively,
between them and the drum 1.
[0077] The elastic layers 402b and 403b of the developing roller
402 and sweep roller 403, respectively, may be formed of a material
other than urethane rubber so long as it does not swell with or
dissolve in the carrier liquid or the developing liquid. Moreover,
the elastic layers 402b and 403b may be simply elastic if the
developing roller 402 and sweep roller 403 each have a conductive
surface and do not swell with or dissolve in the carrier liquid or
the developing liquid and if the internal layers do not contact the
liquid. If the elastic layers 402b and 403b are formed of an
insulator, then a bias voltage for development and a bias voltage,
respectively, must be applied from the surface of the developing
roller 402 and that of the sweep roller 503 instead of from the
cores 402a and 403a.
[0078] In another specific configuration for efficiently forming
the nips between the developing roller 402 and sweep roller 403 and
the drum 1, the rollers 402 and 403 are formed of a rigid material
while an elastic layer is formed on the drum 1. The drum 1 with
elasticity may be replaced with an endless belt, if desired. The
surface of the developing roller 402 and that of the sweep roller
403 are coated, covered with tubes or otherwise processed to have
surface roughness Rz of 3 .mu.m or less.
[0079] Referring again to FIG. 1, when the developing roller 402
and sweep roller 403 are pressed against the drum 1 by adequate
pressure, the elastic layers 502b and 403b elastically deform and
respectively form the development nip and removal nip between the
rollers 402 and 403 and the drum 1. Particularly, the development
nip causes the toner of the developing liquid to be propagated to
and deposited on the drum 1 due to an electric field formed in the
developing zone A, guaranteeing a preselected developing time.
[0080] Further, by controlling the pressure to act between each of
the developing roller 402 and sweep roller 403 and the drum 1, it
is possible to control the width of each nip in the direction in
which the surface of the roller moves. The nip width is selected to
be greater than the product of the linear velocity of each roller
402 or 403 and a development time constant. A development time
constant refers to a period of time necessary for the amount. of
development to saturate and produced by dividing the minimum
necessary nip width by a process speed. For example, if the minimum
necessary nip width is 3 mm while the process speed is 300 mm/sec,
then the development time constant is 10 msec.
[0081] While the developing device 4 is in operation, the
developing liquid is transferred from the coating roller 404 to the
developing roller 402 via the intermediate roller 405, forming a
thin developer layer on the developing roller 402, In the
illustrative embodiment, the thickness of the developer layer on
the developing roller 402 is selected such that the toner contains
a pigment by an amount of 3 .mu.g or above, but 60 .mu.g or below,
for an area of 1 cm.sup.2. For this purpose, the developer layer on
the developing roller 402 is made as thin as 3 .mu.m to 10 .mu.m.
If the pigment content of the toner deposited on the developing
roller 402 for an area of 1 mm.sup.2 is less than 3 .mu.g, then it
is likely that an amount of pigment great enough to insure
sufficient image density cannot be migrated to an image portion
formed on the drum 1. Also, if the pigment content is greater than
60 .mu.g, it is likely that the toner remains on the background of
the drum 1 in an excessive amount after the development of a latent
image, obstructing the removal of toner assigned to the sweep
roller 403.
[0082] The developer layer so formed on the developing roller 402
contacts the drum 1 when brought to the nip for development between
the drum 1 and the developing roller 402, so that the toner is
transferred from the roller 402 to a latent image or image portion
formed on the drum 1. However, the toner does not deposit on the
background or non-image portion of the drum 1, but moves toward the
developing roller 402 due to an electric field formed by a
difference between the bias for development and the potential of
the drum 1.
[0083] If the toner deposited on the background of the drum 1
partly fails to return to the surface of the developer 402 and
remains on the background, then the toner produces fog in the
resulting image. The sweep roller 403 sweeps such toner causative
of fog (fog toner hereinafter). As shown in FIG. 1, the sweep
roller 403 is pressed against the drum 1 at a position downstream
of the developing roller 402 in the direction of rotation of the
drum 1 in such a manner as to sandwich the toner image or developer
layer formed on the drum 1 between it and the drum1. The surface of
the sweep roller 403 moves at substantially the same speed as the
surface of the drum 1 in contact with the drum 1, thereby removing
the fog toner from the background of the drum 1.
[0084] The cleaning member or blade 401 removes the toner left on
the developing roller 402 after the development of the latent image
so as to obviate ghosts. Alternatively, as shown in FIG. 4, there
may be removed the toner transferred from the developing roller 402
to the intermediate roller 405, which rotates in the direction
counter to the developing roller 402. More specifically, in FIG. 4,
a cleaning member or blade 411 is held in contact with the
intermediate roller 405 and removes the toner transferred from the
developing roller 402 to the intermediate roller 405.
[0085] The cleaning member or blade 408 removes the developing
liquid removed by and deposited on the sweep roller 403 in order to
maintain the sweeping function required of the sweep roller
403.
[0086] The developing liquid removed from the rollers 402 through
405, as stated above, is collected in a temporary storage 412
adjoining the reservoir 401, as illustrated. A screw or agitating
means 413 is disposed in the temporary storage 412 for conveying
the developing liquid so collected in the storage 412 to a toner
content adjusting section not shown. After the toner content of the
developing liquid has been adjusted by the toner content adjusting
section, the developing liquid is returned to the reservoir 401 and
used again. Toner content sensing means and liquid level sensing
means, not shown, are also disposed in the temporary storage 412
for respectively sensing the toner content and the liquid level of
the developing liquid collected in the storage 412. The toner
content adjusting section replenishes a fresh developing liquid and
a carrier in accordance with the outputs of the above sensing
means, thereby providing the collected developing liquid with the
preselected toner content. In the illustrative embodiment, the
developing liquid is fed to the reservoir 401 in a slightly greater
amount than it is consumed. As a result, the developing liquid
overflowed the reservoir 401 is returned to the temporary storage
412 and therefore constantly circulated.
[0087] The density or a toner image formed on the drum 1 is
determined by the thickness of the developing liquid or developer
layer deposited on the developing roller 402. In light of this, as
shown in FIG. 5, the coating roller 404 is implemented as, e.g., a
so-called photogravure roller whose surface is formed with cells or
recesses 404a in a regular pattern. FIGS. 6A, 6B and 6C each show a
particular configuration of the cells 404a. As shown, the cells
404a may be implemented as oblique lines (FIG. 6A), pyramids (FIG.
6B) or lattices (FIG. 6C) by way of example. The oblique lines
shown in FIG. 6A are particularly preferable because they promote
desirable image transfer. Excess part of the developing liquid
deposited on the coating roller 404 is squeezed off at the position
where the roller 404 contacts the intermediate roller 405.
Consequently, the developing liquid is deposited on the coating
roller 404 in an amount accurately measured by the recesses
404a.
[0088] The developing liquid so deposited on the coating roller 404
is transferred to the developing roller 402 by way of the
intermediate roller 405, forming a uniform, thin developer layer.
The surface of the intermediate roller 405 moves at the same
peripheral speed and in the same direction as the surface of the
coating roller 404 while moving in the direction counter to the
surface of the developing roller 402, thereby forming the uniform,
thin developer layer on the developing roller 402.
[0089] More specifically, the thin developer layer has the same
pattern as the cells 404a of the coating roller 404 just after it
has been transferred from the coating roller 404 to the
intermediate roller 405. At this instant the surface of the
intermediate roller 405 is moving in the direction counter to the
surface of the developing roller 402. As a result, the developer
layer transferred to the developing roller 402 is drawn out and
leveled due to a difference in linear velocity between the
intermediate roller 405 and the developing roller 402, forming the
uniform, thin layer on the roller 402.
[0090] Assume that the photogravure roller or similar coating
roller 404 formed with the cells 404a and the developing roller 402
directly contact each other. Then, the cells 404a, eliding on the
surface of the developing roller 402, wear and damage the surface
of the roller 402. More specifically, the surface of the developing
roller 402 and that of the coating roller 404 move in opposite
directions to each other, so that the roller 402 contacts the cells
404a with a noticeable difference in linear velocity. As a result,
the surface of the developing roller 402 is damaged, reducing the
life of the roller 402.
[0091] By contrast, in the illustrative embodiment, the developing
liquid is fed from the coating roller 404 to the developing roller
402 byway of the intermediate roller 405, as stated above. The
surface of the developing roller 402 is therefore prevented from
being worn out or damaged by the cells 404a of the coating roller
404. Although the intermediate roller 405 contacts the developing
roller 402 and rotates in the direction counter to the roller 402,
the roller 405 does not damage the surface of the roller 402
because its surface is smooth. This successfully frees the
developing roller 402 from mechanical stress for thereby extending
the life of the roller 402.
[0092] While the intermediate roller 405 contacts the coating
roller 404, the surface of the roller 405 and that of the roller
404 move at the same speed in the same direction, as seen at the
position where they contact each other. Therefore, mechanical
stress exerted by the coating roller 404 on the intermediate roller
405 is so small, the surface of the roller 405 is not worn out or
damaged by the cells 404a of the roller 404.
[0093] The metering member for metering the developing liquid
deposited on the coating roller 404 is generally implemented as the
doctor blade 409 as in the illustrative embodiment or as a doctor
roller. While the coating device using the doctor blade 409 is
relatively simple in construction and can be reduced in size, the
edge of the doctor blade 409 contacting the coating roller 404 is
apt to wear. Further, impurities introduced in the developing
liquid accumulate between the edge of the doctor blade 409 and the
surface of the coating roller 404, making the developer layer
formed on the roller 404 irregular. Although the coating device
using a doctor roller is free from the above problems, such a
coating device is bulky.
[0094] In light of the above, in the illustrative embodiment, the
intermediate roller 405 may be configured to bifunction as a doctor
roller. More specifically, the intermediate roller 405 may be
pressed against the coating roller 404 by pressure high enough to
coat the developing liquid on the roller 404 by an adequately
metered amount.
[0095] FIG. 4 shows a specific configuration of the coating device
in which the intermediate roller 405 bifunctions as a doctor roller
and therefore makes the doctor blade 409, FIG. 1, needless. As
shown, a blade 414 for removing impurities is positioned in the
space that is occupied by the doctor blade 409 in FIG. 1. The blade
414, configured to remove impurities contained in the developing
liquid and deposited on the coating roller 404, is held in contact
with the coating roller 404 with lower pressure than the doctor
blade 409.
[0096] As stated above, in the coating device shown in FIG. 4,
impurities are. prevented from accumulating between the edge of the
blade 414 and the surface of the coating roller 404, so that the
developing liquid can be uniformly coated on the roller 404.
Further, the impurities are leveled at the nip between the coating
roller 404 and the intermediate roller 405 positioned downstream of
the blade 414 in the direction of rotation of the developing roller
404. In this manner, the intermediate roller 405 bifunctions as a
doctor roller and implements a small-size coating device free from
irregular coating.
[0097] In FIG. 4, the cleaning member 407, FIG. 1, assigned to the
developing roller 402 is omitted while the cleaning member 411 is
held in contact with the intermediate roller 405, as stated
earlier. In this condition, the developing liquid left on the
developing roller 402 after development is transferred to the
intermediate roller 405 and then removed from the intermediate
roller 405 by the cleaning member 411. Because the cleaning member
407 is absent, the surface of the developing roller 402 is free
from wear and damage ascribable to the cleaning member 407 and
therefore achieves an extended life.
[0098] The surface of the intermediate roller 405 should preferably
uniformly contact the surface of the coating roller 404 in the
axial direction. This is why the intermediate roller 405 includes
the previously stated elastic layer 405. Also, the intermediate
roller 405 must be pressed against the coating roller 404 by more
than certain pressure when required to bifunction as the metering
member. More specifically, in the illustrative embodiment, the
volume of the cells 403a formed in the coating roller 404
determines the amount of the developing liquid to deposit on the
roller 404. Therefore, if the pressure acting between the
intermediate roller or metering member 405 and the coating roller
404 is short, then the developing liquid is likely to pass through
the nip between the surface of the roller 404 except for the cells
404a and the surface of the roller 405. In such a case, the amount
of the developing liquid actually deposited on the coating roller
404 is short of the expected amount determined by the volume of the
cells 404a, resulting in short image density.
[0099] The amount of the developing liquid to pass through the nip
between the surface of the coating roller 404 except for the cells
404a and the surface of the intermediate roller 405 is dependent on
the linear velocity of the intermediate roller 405 and coating
roller 404 and the viscosity of the developing liquid. For example,
the amount increases with an increase in the viscosity of the
developing liquid. It follows that when the viscosity of the
developing liquid varies due to, e.g., varying ambient temperature,
the amount of the developing liquid to deposit on the coating
roller 404 becomes irregular,
[0100] To solve the above problem, in the illustrative embodiment,
the intermediate roller 405 is pressed against the coating roller
404 by pressure as high as 0.2 Mpa. In this condition, the
intermediate roller 405 bifunctions as a doctor roller capable of
efficiently metering the developing liquid to deposit on the
coating roller 404.
[0101] In principle, the coating roller 404 and intermediate roller
405 each should be a true circle in cross-section so as not to
oscillate. In practice, however, mechanical accuracy available with
this kind of rollers is limited. In the illustrative embodiment,
the elastic layer 405b, FIG. 2, of the intermediate roller 405
absorbs the oscillation of the roller 405, insuring the stable
contact of the roller 405 with the coating roller 404.
[0102] The elastic layer 405b of the intermediate roller 405 should
preferably have rubber hardness of 70.degree. or below in terms of
JIS-A scale.. Rubber hardness higher than 70.degree. makes it
difficult for the intermediate roller 405 to uniformly contact the
coating roller 404. If the intermediate roller 405 is excessively
pressed against the surface of the coating roller. 404, then energy
necessary for the elastic layer 405b to deform increases and
requires the core 405a, which receives the load, to be made thick
and rigid more than necessary. Rubber hardness lower than
30.degree. makes it difficult to implement pressure that allows the
intermediate member 405 to bifunction as the metering member, so
that the elastic layer 405b must noticeably deform and is therefore
reduced in life. For these reasons, in the illustrative embodiment,
the elastic layer 405b is provided with rubber hardness of
40.degree. in terms of JIS-A scale.
[0103] The elastic layer 405b of the intermediate roller 405 may be
formed of urethane rubber or similar rubber. Rubber, however,
generally has a large coefficient of friction and generates
extremely high frictional resistance at the position where the
elastic layer 405b contacts the developing roller 402. Such
frictional resistance not only increases the load on a driveline
assigned to the intermediate roller 405 and developing roller 402,
but also noticeably reduces the life of the rollers 405 and 402.
This is why the smooth layer 405c implemented by a low-friction
member covers the surface of the intermediate roller 405.
[0104] The smooth layer 405c may be implemented by PTFE, PFA, PVDF,
PVF or similar fluorocarbon resin coated on the elastic layer 405b
or a tube formed of fluorocarbon resin and covering the elastic
layer 405b. In the illustrative. embodiment, use is made of a 50
.mu.m thick, PFA tube fitted on the elastic layer 405b. The
developing roller 402 is also covered with a 50 .mu.m thick,
conductive PFA tube.
[0105] A continuous copy test was conducted with the coating device
having the configuration shown in FIG. 4. The test showed that even
after the output of images on 200,000 plain sheets, the developing
roller 402 was free from damage ascribable to the mechanical stress
described above, insuring desirable image quality. Further, when
the photoconductive surface of the drum 1 was formed of a-Si, the
mechanical strength and therefore the life of the drum 1 was
enhanced more than when the above surface was formed of OPC.
[0106] The amount of the developing liquid to deposit on the
developing roller 402 is determined by the volume of the cells 404a
for a unit area and the liquid transfer ratio from the coating
roller 404 to the developing roller 402, as stated above. In the
illustrative embodiment, the coating roller 404 is provided with an
adequate cell volume and pressed against. the intermediate roller
405 by sufficient pressure, thereby preventing the amount of the
developing liquid to deposit on the developing roller 402 from
varying.
[0107] In the above condition, the transfer ratio of the developing
liquid from the coating roller 404 to the developing roller 402 is
expected to remain substantially constant if the properties of the
developing liquid are fixed. However, if, e.g., the viscosity of
the developing liquid varies due to varying ambient temperature,
then the transfer ratio varies. As a result, the density of a toner
image varies if development is effected under the same developing
conditions as before.
[0108] When image density is controlled on the basis of the
strength of an electric field, no problems arise if the electric
field is strengthened for outputting a toner image with high
density. However, when the electric field is weakened to output a
toner image with low density, fine irregularities appear in the
developer layer after development. More specifically, in the weak
electric field, the behavior of toner grains contained in the
developing liquid cannot be controlled. Therefore, at the time when
the surface of the drum 1 and that of the developer 402 part from
each other and cause the developing liquid to split, the two
surfaces pull the split parts of the liquid away from each other.
The resulting traces appear as irregularities in the developer
layer after development. This kind of irregularities are
conspicuous unless the development ratio is 90% or above. It
follows that the control of image density relying on an electric
field is not effective when the amount of the developing liquid
coated varies to such a degree that the development ratio exceeds
10%.
[0109] Moreover, the amount of the developing liquid necessary for
providing a toner image with adequate density is dependent on the
smoothness of the recording medium. For example, a greater amount
of developing liquid is necessary for a plain sheet than for a
coated sheet or similar highly smooth recording medium in order to
fill up the irregularities of the plain sheet.
[0110] To solve the above problems, as shown in FIG. 9, the
illustrative embodiment includes speed varying means 415 capable of
varying the rotation speed of the coating roller 404 and that of
the intermediate roller 405 while maintaining the two rotation
speeds equal to each other. More specifically, the speed varying
means 415 varies the rotation speed of the coating roller 404 and
that of the intermediate roller 405 to thereby vary the relative
moving speed of the surface of the intermediate roller 405 and that
of the developing roller 402.
[0111] As shown in FIG. 8, the amount of the developing liquid to
be coated on the developing roller 402 by the intermediate roller
405 substantially linearly varies in proportion to the speed ratio
between the developing roller 402 and the intermediate roller 405.
The illustrative embodiment can therefore control the above amount
by varying the speed ratio between the developing roller 402 and
the intermediate roller 405.
[0112] As shown in FIG. 9 specifically, a driveline for driving the
coating roller 404 and intermediate roller 405 includes drive
transmitting means 416 implemented by, e.g., gears and a timing
belt. A servo motor, stepping motor or similar motor 417 drives the
coating roller 404 and intermediate roller 405 via the drive
transmitting means 416. More specifically, a driver 418 drives the
motor 417 at a rotation speed matching with a frequency signal and
an AID (Analog/Digital) signal, which a controller 419 outputs in
accordance with the characteristics of the motor 417.
[0113] The controller 419 generates adequate signals in accordance
with a request input from an operation panel 420 and a control
board 421, thereby controlling the rotation speed of the
intermediate roller 405 and that of the coating roller 404.
Consequently, the linear speed ratio of the intermediate roller 405
and coating roller 404 to the developing roller 402 is controlled,
so that the amount of the developing liquid to deposit on the
developing roller 402 is controlled.
[0114] If only the peripheral speed of the intermediate roller 405
is varied, then a difference in peripheral speed occurs between the
coating roller 404 and the intermediate roller 405 and causes the
cells 404a of the roller 404a to scratch the surface of the roller
405, thereby reducing the life of the roller 405. It is therefore
preferable to obviate the above difference as far as possible. This
is why the illustrative embodiment varies the speed ratio between
the developing roller 402 and the intermediate roller 405 while
maintaining the peripheral speed of the coating roller 404 and that
of the intermediate roller 405 the same. At this instant, a single
drive source should preferably drive both of the coating roller 404
and intermediate roller 405. If the two rollers 404 and 405 each
are driven by a respective drive source, then it is difficult to
match the peripheral speed of the roller 404 and that of the roller
405 and therefore to obviate a difference between them.
[0115] FIG. 10 shows a specific drive mechanism in which a single
drive source drives both of the coating roller 404 and intermediate
roller 405 while maintaining their peripheral speeds the same. As
shown, the motor 417 causes the intermediate roller 405 to rotate
in a direction indicated by an arrow via the drive transmitting
means 416. The intermediate roller 405, in turn, causes the coating
roller 404 to rotate in a direction indicated by an arrow with a
frictional force acting therebetween. This successfully drives both
of the rollers 404 and 405 at the same speed.
[0116] While the drive mechanism of FIG. 10 may be modified to
cause the coating roller 404 to drive the intermediate roller 405
with a frictional force, this kind of scheme gives rise to the
following problem. The intermediate roller 405 contacts the
developing roller 402 to be coated with the developing liquid.
This, coupled with the fact that the surface of the intermediate
roller 405 moves counter to the surface of the developing roller
402, makes the rotation of the roller 405 unstable due to a load
torque acting on the roller 405. In this respect, the drive
mechanism of FIG. 10 frees the coating roller 404 from the load of
the developing roller 402 for thereby insuring the stable drive of
the rollers 404 and 405.
[0117] The developing liquid is made up of a solvent implemented by
dimethylpolysiloxane oil or similar insulative liquid and toner
grains densely dispersed in the liquid. Therefore, when the
intermediate roller 405 causes the coating roller 404 to rotate
with the intermediary of the developing liquid deposited on the
roller 404, it is likely that the developing liquid functions as a
lubricant and causes the roller 404 to slip on the roller 405.
Particularly, when the intermediate roller 405 is driven at high
speed, the slip of the coating roller 404 is apt to render the
amount of the developing liquid to deposit on the roller 405
unstable.
[0118] FIG. 11 shows another specific configuration of the drive
mechanism configured to obviate the slip of the intermediate roller
405 despite the use of a single drive source. As shown, the motor
417 drives not only the intermediate roller 405 but also the
coating roller 404 via the drive transmitting means 416. The
peripheral speed of the coating roller 404 is selected to be
slightly lower than the peripheral speed of the intermediate roller
405, i.e., the former is lower than the latter by 2% to 5% in the
illustrative embodiment. A one-way clutch 423 is included in part
of the drive transmitting means 416 assigned to the coating roller
404. The one-way clutch 423 transmits the rotation of the motor 417
to the coating roller 404 in a direction indicated by an arrow, but
does not transmit it to the roller 404 in the other direction.
[0119] In FIG. 11, when the motor 416 is rotated, the drive
transmitting means 416 causes the intermediate roller 405 to rotate
in the direction indicated by the arrow. So long as the coating
roller 404 does not slip, the torque of the motor 417 is not
transferred to the coating roller 404. Consequently, the
intermediate roller 405 causes the coating roller 404 to rotate at
the same speed as the roller 405 as in the drive mechanism of FIG.
10. More specifically, in this condition, the one-way clutch 423
causes the coating roller 404 to simply idle and follow the
rotation of the intermediate roller 405, so that the coating roller
404 is free from a braking force.
[0120] On the other hand, when the coating roller 404 slips, it is
not driven by the intermediate roller 405, but is caused to rotate
by the drive transmitting means 416 at a slightly higher peripheral
speed than the intermediate roller 405 in the direction indicated
by the arrow. In this manner, the drive mechanism of FIG. 11
prevents the amount of the developing liquid coated from varying
due to the slip of the coating roller 404. In addition, the drive
mechanism reduces the difference in peripheral speed between the
intermediate roller 405 and the coating roller 404, thereby
protecting the surface of the roller 405 from damage ascribable to
the above difference.
[0121] Assume a pair of rollers configured to move in the same
direction at a position where they contact each other. Then, the
amount of a liquid passing through the nip between the rollers and
the split ratio of the liquid on the rollers after the passage of
the liquid are generally dependent, on the peripheral speed,
geometric configuration of the surface and material constant of
each roller as well as on the viscosity of the liquid. More
specifically, it is well known that at the time when the coated
liquid splits into two parts respectively deposited on the two
rollers, the liquid unevenly splits in the form of rings along the
circumference of the rollers.
[0122] To solve the above problem, the intermediate roller 405 of
the coating device should preferably be so driven as to move in the
opposite direction to the developing roller 402 where the former
contacts the latter. However, in the illustrative embodiment, the
coating roller 404 is formed with the cells 404a and therefore
transfers the developing to the intermediate roller 405 in the same
pattern as the cells 404a. Therefore, if the peripheral speed Vm of
the intermediate roller 405 and the peripheral speed Vd of the
developing roller 402 are the same (Vm.ltoreq.Vd), then the
developer layer identical in pattern with the cells 404a is
directly transferred to the developing roller 402 in a size reduced
in accordance with the speed ratio between the rollers 405 and 402.
This makes it impossible for the developing liquid to form the
desired uniform, thin developer layer on the developing roller
402.
[0123] In light of the above, in the illustrative embodiment, it is
necessary to make the peripheral speed Vm of the intermediate
roller 405 higher than the peripheral speed Vd of the developing
roller 402 for thereby leveling the developer layer transferred to
the roller 402 in the pattern .of the cells 404a. While the amount
of the developing liquid to deposit on the developing roller 402
substantially linearly varies in accordance with the ratio Vm/Vd,
as shown in FIG. 8, the transfer of the developer layer to the
roller 402 becomes unstable when the peripheral speed Vm is
increased to about two times as high as the peripheral speed Vd. In
the illustrative embodiment, the peripheral speed Vm of the
intermediate roller 405 is selected to lie in the range of
Vd<Vm<2.times.Vd. In this condition, the intermediate roller
405 can level the developer layer transferred to the developing
roller 402 in the pattern of the cells 404a, thereby forming the
uniform, thin developer layer. In addition, the developer layer can
be stably transferred to the developing roller 402 in a constant
amount.
[0124] As stated above, in the illustrative embodiment, the
intermediate roller 405 moves in the same direction as the coating
roller 404, but moves in the opposite direction to the developing
roller 402. The coating roller 404 and developing roller 402
therefore sandwich the intermediate roller 405 therebetween. In
this condition, assume that the intermediate roller 405 contacts
the coating roller 404 and developing roller 402 at a position
downstream of a line virtually connecting the axes of the rollers
404 and 402 in the direction of rotation of the rollers 404 and
402. Then, a force tending to force the intermediate roller 405
away from the coating roller 404 and developing roller 402 acts on
the intermediate roller 405. It is therefore likely that contact of
the intermediate roller 405 with the coating roller 404 and
developing roller 402 is unstable, preventing the developing liquid
from being stably coated on the developing roller 402.
[0125] FIG. 12 shows a solution to the above problem. As shown, the
intermediate roller 405 contacts the coating roller 404 and
developing roller 402 at a position upstream of the line virtually
connecting the axes of the rollers 404 and 402. More specifically,
a line tangential to the intermediate roller 405 and developing
roller 402 is located at a position upstream of, in the direction
of rotation of the roller 405, a line where a plane containing the
axes of the rollers 404 and 405 and the surface of the roller 405
intersect each other and including the above line. In this
configuration, a force tending to nip the intermediate roller 405
between the coating roller 404 and the developing roller 402 acts
on the intermediate roller 405 due to the rotation of the rollers
404 and 402. Such a force stabilizes contact of the intermediate
roller 405 with the coating roller 404 and developing roller 402,
insuring stable coating of the developing liquid on the roller
402.
[0126] With the coating device described above, it is possible to
correct the amount of the developing liquid coated on the
developing roller 402 when it varies due to the variation of the
transfer ratio of the liquid to the intermediate roller 405. This
kind of variation is likely to occur when, e.g., temperature inside
the image forming apparatus varies and causes the viscosity of the
developing liquid to vary. It is also possible to adjust the amount
of the developing liquid coated when the toner content of the
developing liquid varies. In any case, by so controlling the amount
of the developing liquid, it is possible to maintain image density
constant. For this kind of control, it is necessary to sense image
density. Usually, an optical sensor capable of optically sensing
image density is used for this purpose.
[0127] The optical sensing means should preferably sense image
density at the last image forming stage as far as possible in order
to maintain image density constant. However, it is difficult to
form an exclusive pattern image for image density sensing on a
sheet carrying a toner image thereon. Generally, therefore, a
pattern image is formed on the non-image area of the belt 502 or
that of the drum 1, so that the sensor can sense the density of the
pattern image, see FIG. 1.
[0128] If it suffices to maintain only the amount of the developing
liquid to deposit on the developing roller 402 constant, then the
toner content of the developer coated on the roller 402 should only
be sensed. However, because developing and image transferring
conditions also effect image density, it is more preferable to
sense the density of the pattern image formed on the belt 502 or
the drum 1 with the image density sensor.
[0129] FIG. 13 shows another specific configuration of the coating
device using a density sensor 424 for sensing the image density of
the toner image formed on the belt 502 or the drum 1. The density
sensor 424, implemented as an optical sensor by way of example, is
positioned in the vicinity of the belt 502 or the drum 1.
Information representative of image density is fed from the image
sensor 424 to the control board 421 in addition to image density
information input on the control panel 420 by the operator. The
control board 421 compares the two kinds of image density
information and then determines whether or not the amount of the
developing liquid to deposit on the developing roller 402 should be
varied. Subsequently, the control board 421 determines the rotation
speed of the motor 417 in accordance with the result of the above
decision and feeds signals to the controller 419 and driver 418,
thereby controlling the motor 417. As for the above decision,
adequate density information determined beforehand may
alternatively be stored in a memory 422, in which case the control
board 421 will compare the output of the density sensor 424 with
the stored information.
[0130] In the copier described with reference to FIG. 1 or 4, a
single developing device 4 adjoins the drum 1. If four developing
devices storing yellow, magenta, cyan and black developing liquids,
respectively, are arranged around the drum 1, then there can be
implemented a color copier capable of forming a full-color toner
image on the drum 1 as conventional. It is desirable with the color
copier to control the amount of coating liquid by liquid because
each liquid has particular properties including viscosity
susceptible to varying ambient temperature. Of course, the number
of colors is not limited to four, but may be two or five, if
desired.
[0131] The coating device capable of controlling the speed of the
coating roller 404 and that of the intermediate roller 405, as
stated earlier, not only stabilizes image quality by reducing the
variation of image density, but also allows the developing liquid
to be coated in an amount optimum for the kind of a sheet used.
More specifically, the operator inputs the kind of a sheet to use
on the operation panel 420 or the control board 421 determines the
kind of sheets stacked on a designated sheet cassette. In this
case, rotation speeds of the motor 417 each implementing an optimum
amount of developing liquid for a particular kind of sheet are
stored in the memory 422. The control board 421 selects the optimum
amount of developing liquid matching with the sheet to be used. The
control board 421 then sends signals matching with the rotation
speed selected to the controller 419 and driver 418, thereby
controlling the rotation speed of the motor 417. Consequently, the
developing liquid is deposited on the developing roller 402 in an
amount optimum for the kind of the sheet.
[0132] Further, the operation panel 420 maybe provided with a
function that allows the operator to select desired image density,
in which case the developing liquid will be deposited on the
developing roller 402 in an optimum amount matching with the
desired image density. The color copier can therefore control the
tone of a color image without varying image data itself.
[0133] As shown in FIG. 14, in the illustrative embodiment, the
coating roller 404 is provided with a greater width than the
intermediate roller 405 in the axial direction, so that opposite
end portions of the roller 404 do not contact the roller 405. As
shown in FIGS. 15A and 15B, if such a relation between the coating
roller 404 and the intermediate roller 405 is reversed, then excess
part of the developing liquid squeezed out from between the rollers
404 and 405 deposits on the roller 405 and is transferred from the
roller 405 to the developing roller 402. Consequently, this part of
the developing liquid is transferred to a sheet via the drum 1 and
belt 502, resulting in the degradation of image quality and
wastefully consumption of the liquid.
[0134] In the configuration shown in FIG. 14, excess part of the
developing liquid squeezed out from between the coating roller 404
and the intermediate roller 405 deposits on the roller 404. Stated
another way, only the adequate amount of developing liquid moved
away from the nip between the coating roller 404 and the
intermediate roller 405 deposits on the intermediate roller 405,
obviating the problems stated above. In addition, the intermediate
roller 405 with the elastic layer 405b is prevented from being
scratched by the end portions of the coating roller 404, which is a
metallic roller.
[0135] Assume that the developer roller 402 has a greater width
than the intermediate roller 405, which forms the thin developer
layer on the developing roller 402. Then, as shown in FIG. 16, the
end portions of the intermediate roller 405 contact the developing
roller 402. As a result, the developing liquid squeezed out of the
nip between the roller 405 and the coating roller 404 and deposited
on the end faces of the roller 405, although in a small amount, is
transferred to the developing roller 402. The developing liquid so
deposited on the developing roller 402 is extended in the axial
direction at the nip between the roller 402 and the drum 1. This
part of the developing liquid cannot be controlled by the electric
field for development, but deposits on the drum 1, producing fog in
the non-image portion and irregular density in the image portion.
Although the developing roller 402 may be provided with a greater
width than the drum 1 in order to prevent the end portions of the
roller 402 from contacting the drum 1, this kind of scheme causes
the developing roller 402 with the elastic layer 402b to contact
the rigid drum 1 and scratch it.
[0136] It is difficult to uniformly charge the entire drum 1 in the
axial direction, so that non-charged portions or portions with
short charge usually exist at opposite end portions of the drum 1.
Taking this into consideration, it is a common practice with this
type of copier to determine a valid charging range and a valid
image range. On the other hand, if the developing roller 402
contacts the portions with short charge, then the developing liquid
is transferred from the developing roller 402 to the drum 1 because
it cannot be controlled by the electric field for development. It
follows that the width of the developing roller 402 must be smaller
than the valid charging range of the drum 1, but greater the width
of the valid image range.
[0137] For the reasons stated above, as shown in FIG. 14, the
intermediate roller 405 is made longer than the developing roller
402. This prevents the developing liquid deposited on the end faces
of the intermediate roller 405 from being transferred to the
developing roller 402.
[0138] In the developing device of the type developing a latent
image formed on the drum 1 with the thin developer layer deposited
on the developing roller. 402, the developing liquid is, in many
cases, partly left on the developing roller 402 after development.
The solid content of such residual part of the developing liquid
differs from the original solid content.
[0139] More specifically, in part of the developing liquid
corresponding to the latent image or image portion, the solid is
deposited on the drum 1 while the carrier is left on the developing
roller 402. alone. On the other hand, in part of the developing
liquid corresponding to the non-image portion, the carrier is
partly deposited on the drum 1 while substantially the entire
solid, i.e., the dense developing liquid is left on the developing
roller 402. If the developing liquid is again coated on the
developing roller 402 for the next development over the residual
developing liquid, then a density difference occurs in the thin
developer layer coated on the developing roller 402, resulting in
the ghost of the previous image. It is therefore necessary to
remove the developing liquid used for the previous development from
the developing roller 402 before again coating the developing
liquid on the roller 402.
[0140] It is a common practice with the developing device to remove
the residual developing liquid from the developing roller 402 by
use of a blade held in contact with the surface of the roller 402.
The blade is formed of rubber or implemented as a laminate of
rubber and metal.
[0141] In the illustrative embodiment, the surface of the
intermediate roller 405 contacts the surface of the developing
roller 402 in such a manner as to move in the opposite direction to
the developing roller 402, as seen at the position where the former
contacts the latter, as stated earlier. The intermediate roller 405
can therefore clean the surface of the developing roller 402 and
makes it needless to use an exclusive cleaning member. The
exclusive cleaning member might damage the surface of the
developing roller 402 and would increase the load on the drive of
the roller 402.
[0142] On the other hand, if part of the developing liquid left on
the intermediate roller 405 after the transfer of the liquid to the
developing roller 402 is not removed, then the residual developing
liquid is again fed to the developing roller 402 via the nip
between the rollers 405 and 402, resulting in the ghost stated
above. In the illustrative embodiment, as shown in FIG. 4, the
cleaning member 411 held in contact with the intermediate roller
405 removes the residual developing liquid from the roller 405.
[0143] More specifically, the cleaning member 411 is implemented as
a rubber blade. The developing liquid removed by the cleaning
member 411 is collected in the temporary storage 412 without being
mixed with the adjusted developing liquid present in the reservoir
401. The collected developing liquid is then conveyed by the screw
413 to the density adjusting section, adjusted in toner content,
and then returned to the reservoir 401. In this manner, the
cleaning member 411 can remove the residual developing liquid from
the developing roller 402 via the intermediate roller 405, thereby
obviating the ghost.
[0144] As shown in FIGS. 17A and 17B, the cleaning member 411
should preferably be greater in width than the developing roller
402 in the axial direction in order to remove all the developing
liquid left on the developing roller 402 after development. Should
the developing liquid left on the developing roller 402 and not
adjusted in toner content be again used for development, it would
bring about various image detects including an irregular density
distribution.
[0145] Further, if the cleaning member or blade 411 is smaller in
width than the intermediate roller 405 in the axial direction, then
stress concentrates on the surface portion of the intermediate
roller 405 contacting the edge of the cleaning member 411 and is
likely to damage the above surface portion. In this respect, the
cleaning member 411 should preferably be greater in width than the
intermediate roller 405, see FIG. 17A. By so limiting the width of
the cleaning member 411, it is possible to prevent the residual
developing liquid left on the developing roller 402 from being
again used without adjustment.
[0146] The developing roller 404 formed with the cells 404a in a
uniform pattern over its entire circumference may be replaced with
a plain roller not formed with the cells 404a, i.e., having a
smooth surface. When a plain roller is replaced with the developing
roller 404, the amount of the developing liquid to pass through the
nip between the coating roller 404 and the intermediate roller 405
is determined by pressure to act between the rollers 404 and 405,
geometric configurations of the rollers 404 and 405, longitudinal
elasticity constants of the rollers 404 and 405, viscosity of the
developing liquid, and rotation speeds of the rollers 404 and 405.
The distribution ratio of the developing liquid to the rollers 404
and 405 is determined by the speed ratio between the rollers 404
and 405.
[0147] However, it is difficult to uniform the pressure acting
between the plain roller and the intermediate roller 405 in the
axial direction because of limited mechanical, dimensional accuracy
and uniformity of material constants. The irregular pressure
distribution between the plain roller and the intermediate roller
405 would cause the amount of the developing liquid transferred
from the plain roller to the intermediate roller 405 to vary.
[0148] By contrast, the coating roller 404 of the illustrative
embodiment can accurately measure the developing liquid to be
applied to the coating roller 404 on the basis of the cell volume
of the cells 404a, as stated earlier. In the illustrative
embodiment, the cells 404a that need high dimensional accuracy are
formed on the surface of the coating roller 404 by transfer.
Transfer is advantageous over machining because it can
substantially faithfully reproduce the shape of a die and can cause
a surface to solidify by plastic deformation. However, transfer is
not feasible for materials having hardness of 200 HV or above. A
surface needing hardness is plated with hard chromium or otherwise
treated.
[0149] When the cells 404a are formed in the entire surface of the
coating roller 404, the developing liquid is transferred from the
roller 404 to the entire surface of the intermediate roller 405. As
shown in FIG. 17A, the developing roller 402 is smaller in width
than the intermediate roller 405 in the axial direction.
Consequently, as shown in FIG. 19, when the developing liquid is
transferred from the intermediate roller 405 to the developing
roller 402, excess part of the developing liquid, although small in
amount, squeezed out from the nip between the rollers 402 and 405
in the axial direction deposits on the end of the roller 402 and is
transferred from the roller 402 to the drum 1. This part of the
developing liquid is further transferred from the drum 1 to the
belt 502 and therefore to a sheet while increasing the load on the
cleaning device 6 assigned to the drum 1.
[0150] To solve the above problem, as shown in FIG. 19, the coating
roller 404 is not formed with the cells 404a in its opposite end
portions 404b. In this configuration, hardly any developing liquid
is transferred to the portions of the intermediate roller 405
facing the end portions or non-carved portions 404b of the coating
roller 404. Stated another way, the developing liquid is
transferred only to the portion of the intermediate roller 405
facing the intermediate portion of the coating roller 404 formed
with the cells 404a.
[0151] As shown in FIG. 19, the intermediate roller 405 is provided
with a greater width than the portion of the coating roller 404
formed with the cells 404a (coating range) in the axial direction,
so that the developing liquid does not deposit on the opposite end
portions of the intermediate roller 405.
[0152] Also, the developing roller 402 is provided with a greater
width than the above coating range of the coating roller 404 in the
axial direction such that opposite end portions of the developing
roller 402 contact the end portions of the intermediate roller 405.
This successfully obviates the deposition of the developing liquid
on the end of the developing roller 402. At this instant, it is
necessary to make the coating range of the coating roller 404 wider
than the valid image range of the drum 1 and to make the valid
charging range of the drum. 1 slightly wider than the coating range
of the coating roller 404. If the valid charging range is
excessively wide, then the surface portions of the developing
roller 402 not coated with the developing liquid contact the drum 1
and render the electric field formed in the developing zone A
unstable.
Second Embodiment
[0153] Referring to FIG. 20, an alternative embodiment of the
present invention also implemented as an electrophotographic copier
will be described hereinafter. As shown, as for basic construction,
the illustrative embodiment is generally identical with the
previous embodiment. The following description will concentrate on
features unique to the illustrative embodiment in order to avoid
redundancy.
[0154] The surface of the intermediate roller 405 and that of the
developing roller 402 move in opposite directions to each other at
the position where they contact each other, as stated previously.
Therefore, if the elastic layer of the intermediate roller 405
having a large coefficient of friction is exposed to the outside,
then extremely high frictional resistance is generated at the above
contact position. Such frictional resistance not only increases the
load on the driveline assigned to the intermediate roller 405 and
developing roller 402, but also noticeably reduces the life of the
rollers 405 and 402. In light of this, a smooth layer with a small
coefficient of friction covers the surface of the intermediate
roller 405. Again, the smooth layer may be implemented by PTFE,
PFA, PVDF, PVF or similar fluorocarbon resin coated on the
intermediate roller 405 or a tube formed of fluorocarbon resin and
covering the roller. In the illustrative embodiment, as shown in
FIG. 21, use is made of a 50 .mu.m thick, PFA tube 405C fitted on
the intermediate roller 405. The developing roller 402 is also
covered with a 50 .mu.m thick, conductive PFA tube.
[0155] When use was made of a PFA tube whose carbon content was
reduced for enhancing the mechanical strength of the surface of the
intermediate roller 405, the developing liquid was scattered away
from the coating surface of the roller 405. It was experimentally
found that when the surface layer of the intermediate roller 405
had high resistance, frictional charge occurred between the
developing roller 402 and the cleaning blade 411 and caused the
developing liquid to be scattered around. FIG. 22 shows the results
of a series of experiments conducted by varying the resistance of
the surface layer. As shown, so long as the surface resistance of
the intermediate roller 405 is 10.sup.13 .OMEGA./cm.sup.2 or below,
the developing liquid is prevented from being scattered around.
[0156] More specifically, as shown in FIG. 22, sample Nos. 1
through 6 of the intermediate roller 405 respectively having
surface resistances R of R>10.sup.13 .OMEGA./cm.sup.2, 10.sup.12
.OMEGA./cm.sup.2, 10.sup.11 .OMEGA./cm.sup.2, 10.sup.10
.OMEGA./cm.sup.2 and 10.sup.6 .OMEGA./cm.sup.2 were prepared and
estimated as to durability. Only the sample No. 6 whose surface
resistance R was higher than 10.sup.13 .OMEGA./cm.sup.2 caused the
developing liquid to be scattered away from the intermediate roller
405 due to the charge. This is why the surface resistance of the
intermediate roller 405 should preferably be 10.sup.13
.OMEGA./cm.sup.2 or below. While the lower limit of the surface
resistance is not definite as to function, it is about 10.sup.5
.OMEGA./cm.sup.2 for production reasons.
[0157] Referring again to FIG. 20, the developing roller 402 is
applied with the bias for forming the electric field in the
developing zone between the roller 402 and the drum 1. The
intermediate roller 405 is held in contact with the developing
roller 402 and coating roller 404, which is a metallic roller.
Therefore, the voltage applied to the developing roller 402 leaks
and cannot form the electric field unless the surface layer of the
intermediate roller 405 is electrically insulative. It follows that
the coating roller 404 and cleaning blade 414 must be electrically
insulated from the body that supports them. Further, if the coating
roller 404 and cleaning blade 414 are electrically floating, then
they are charged and effect the electric field. To solve this
problem, in the event of image formation, a voltage is applied to
the intermediate roller 405 such that the surface of the roller 405
is of the same potential as the surface of the developing roller
402, thereby insuring a stable image. To set the potential of the
intermediate roller 405, a terminal may be held in contact with the
shaft of the intermediate roller 405 if the inner layer of the
roller 405 is conductive. If the inner layer is insulative, then
the voltage may be applied to the intermediate roller 405 via the
coating roller 404.
[0158] The developing roller or developer carrier 402 is pressed
against the drum or image carrier 1 with its elastic layer being
deformed, so that the nip for development is. formed between the
roller 402 and the drum 1. At this instant, a sufficient nip is
insured by, e.g., making the hardness of the elastic layer low. For
this purpose, in the illustrative embodiment, the elastic layer of
the developing roller 402 is formed of conductive urethane resin
having hardness of 25.degree. in terms of JIS-A scale. Generally,
to provide the elastic layer with low hardness, resin containing,
e.g., oil is used. This kind of scheme, however, causes the shape
of the elastic layer to vary when the elastic layer left in the
deformed condition over a long period of time. If the deformation
of the elastic layer is not small, then the elastic layer can
restore its original shape after the developing roller 402 has been
rotated for a while. However, the elastic layer undergoes permanent
set if its deformation is great and if the deformed condition lasts
over a long period of time in a hot environment. The so deformed
developing roller 402 causes the amount of the developing roller to
deposit thereon to vary in accordance with the deformation,
critically lowering image quality. It is wasteful to replace the
deformed developing roller 402 before its life ends.
[0159] In light of the above, as shown in FIG. 23, the illustrative
embodiment includes a moving mechanism for selectively moving the
developing roller 402 and drum 1 into or out of contact with each
other. When image formation is not under way, the moving mechanism
releases the developing roller 402 from the drum 1 and thereby
prevents the roller 402, which is in a halt then, from remaining in
pressing contact with the drum 1 over a long period of time.
[0160] FIGS. 24 and 25 show a specific configuration of the moving
means mentioned above. As shown, the reservoir 401, supporting the
developing roller 402, is angularly movable about a fulcrum 450a
provided on a press arm 450. A spring or biasing means 451 is
anchored to the press arm 450 at one end and constantly biases the
shaft 402a of the developing roller 402 or a member supporting the
shaft 402a such that the roller 402 tends to contact the drum 1. A
cam follower 450b is mounted on the free end of the press arm 450
and held in contact with the edge of an eccentric cam 452 by the
force of the spring 451. The cam 452 causes the press arm 450 to
angularly move about the fulcrum 450a when driven to rotate. The
movable range of the press arm 450 is delimited by the eccentricity
of the cam 452 and a stop 453. The biasing force of the spring 451
is adjustable via a screw 454.
[0161] While the condition in which the developing roller 1 is
pressed against the drum 1 is determined by the eccentricity of the
cam 452, as stated above, the condition may alternatively be
determined by the biasing force of the spring 451. The problem with
the eccentricity scheme is that any different in the eccentricity
of the cam 452 translates into a noticeable change in the condition
of contact of the developing roller 402 with the drum 1, resulting
in the need for extremely accurate control. In light of this, the
moving mechanism may press the developing roller 402 against the
drum 1 with the biasing force of the spring 451 and release the
former from the latter with the eccentricity of the cam 452, so
that the cam 452 with relatively low accuracy can implement a
preselected pressing condition.
[0162] In FIGS. 24 and 25, the moving mechanism causes the
developing roller 402 and coating device to bodily move about the
fulcrum 450a. Alternatively, the moving mechanism may be modified
to move the coating device horizontally or move only the developing
roller 402.
[0163] The coating device with the developing roller 402 is
arranged in the reservoir 401, which is a substantially
hermetically sealed container except for the portion where the
roller 402 contacts the drum 1. Therefore, a minimum of impurities
is allowed into the reservoir 401; otherwise, impurities would be
introduced in the developing liquid and would make coating of the
liquid defective, produce stripes, and damage or wear the surfaces
of the developing roller 402 and intermediate roller 405 by
blocking their nip.
[0164] FIG. 26 shows a specific configuration of the hermetically
sealed reservoir 401. As shown, the developing roller 402,
intermediate roller 405, coating roller 404 and screw 406 are
accommodated in the reservoir 401. The reservoir 401 is formed with
an inlet port 401c for feeding the developing liquid D to the
reservoir 401 and an outlet port 401b for discharging the
developing liquid removed from the rollers 402 through 405.
[0165] A pump 461 delivers the developing liquid with the adjusted
toner content from a density adjusting device 460 to the reservoir
401 via the inlet port 410a. The screw 406 conveys the incoming
developing liquid D while agitating it such that the liquid D is
distributed mainly in the lengthwise direction of the reservoir
401.
[0166] The reservoir 401 is divided into an inlet chamber and an
outlet chamber. The coating roller 404 is immersed in the
developing liquid stored in the inlet chamber. The developing
liquid deposited on the coating roller 404 is metered by the
coating roller 404 and intermediate roller 405 and then coated on
the developing roller 402 by the intermediate roller 405 in a thin
layer. The developing liquid left on the developing roller 402
after development is removed by the intermediate roller 405 and
then collected by the cleaning blade 411 contacting the roller 405.
The developing liquid collected by the cleaning blade 411 should
not be directly used for the next development because its toner
content has varied. The developing liquid is therefore once stored
in the outlet chamber, delivered to the density adjusting section
460 via the outlet port 401b, and then returned to the reservoir,
401 via the inlet port 410c. Part of the developing liquid
overflowed the inlet chamber is introduced in the outlet chamber in
the same manner as in the previous embodiment, maintaining the
amount of the developing liquid in the reservoir 401 constant.
[0167] The hermetically sealed reservoir 401 must be formed with an
opening 401a for allowing the developing roller 402 to contact the
drum 1. Therefore, in the coating device constructed to be
angularly movable together with the developing roller 402, the
opening 401a is widely opened when the developing roller 402 is
released from the drum 1. In this condition, impurities are likely
to enter the coating device via the opening 401a. To solve this
problem, a shutter 470 should preferably selectively open or close
the opening 401a. When the developing roller 402 is spaced from the
drum 1, e.g., when image formation is not under way, the shutter
470 closes the opening 401a to thereby prevent impurities from
entering via the opening 401a.
[0168] FIG. 27 shows a specific configuration of a mechanism for
opening and closing the shutter 470. As shown, the shutter 470 is
mounted on the reservoir 401 in such a manner as to be pivotable to
open or close the opening 401a. The pivotal portion of the shutter
470 is implemented as a gear 471 held in mesh with a drive gear
472, which is interlocked to the cam 452 of the moving mechanism.
In this configuration, when the developing roller 402 moves into or
out of contact with the drum 1 in accordance with the rotation of
the cam 452, the drive gear 472 causes the shutter 470 to open or
close, respectively, the opening 401a of the reservoir 401 via the
gear 471. If desired, the shutter 470 may be caused to open and
close the opening 401a by a spring or a rotatable cam.
[0169] Although the shutter 470 prevents impurities from entering
the reservoir 401 via the opening 401a, it is likely that
impurities are contained in the developing liquid newly fed to the
reservoir 401 via the inlet port 401c. To cope with such
impurities, as shown in FIG. 26, an impurity removing device or
means 480 should preferably be positioned upstream of the inlet
port 401c. The impurity removing device 480 includes a filter 482
for filtering out paper dust and other nonmetallic impurities and a
magnet 483. The filter 482 is configured in parallel to the stream
of the developing liquid because its area and therefore life would
be reduced if positioned perpendicularly to the flow. Also, the
filter 482 should preferably be disposed in a conduit providing
communication between the density adjusting device 460 and the
reservoir 401. In the illustrative embodiment, an arrangement is
made such that the developing liquid efficiently flows through the
filter 462 because of the pressure of the pump 461. In addition,
the filter 482 so configured and arranged is easy to replace or
clean periodically.
[0170] The surface of the developing roller 402 moves in the
opposite direction to the surface of the intermediate roller 405,
as stated earlier. In this condition, as shown in FIG. 28, the
developing liquid is subjected to an intense shearing force at and
around the inlet and outlet of the nip between the developing
roller 402 and the intermediate roller 405, so that unstable zones
a and b appear in the thin developer layer. Should impurities enter
the unstable zone a or b, they cannot escape from it and form
continuous stripes on the coated surface of the developing roller
402. Such impurities entered the unstable zone a, or b can be
effectively removed if the developing roller 402 and intermediate
roller 405 are released from each other. For this purpose, a moving
mechanism should be provided that. selectively moves the developing
roller 402 and intermediate roller into or out of contact with each
other 405.
[0171] FIGS. 39 and 30 show a specific configuration of the moving
mechanism that moves only the developing roller 402 into and out of
contact with the drum 1. As shown, the moving mechanism, generally
600, includes a press arm 602 angularly movable about a shaft 601.
A spring or biasing means 603 constantly biases the press arm 602
such that the developing roller 402 tends to move toward the drum
1. A cam 604 is pressed against a cam follower mounted on the press
arm 602. When the cam 604 is rotated, it causes the press arm 602
to angularly move about the shaft 601. The biasing force of the
spring 603 is adjustable via a screw 605.
[0172] As stated above, the moving mechanism 600 for moving only
the developing roller 402 is provided independently of the moving
mechanism assigned to the entire coating device and can move the
roller 402 into or out of contact with the intermediate roller 405
only if the strokes of the two moving means are slightly different
from each other. By releasing the developing roller 402 from the
intermediate roller 405 periodically, e.g., before the start of
image formation or when every 1,000 copies are continuously output,
it is possible to obviate the degradation of image quality
ascribable to impurities accumulated at the nip between the rollers
405 and 402.
[0173] However, the problem with the moving mechanism 600 is that
it occupies an additional space and makes the developing device 4
bulky while making the operation sequence thereof sophisticated. It
is therefore preferable to interlock the moving mechanisms assigned
to the entire coating device and developing roller 402,
respectively.
[0174] FIGS. 31 and 32 show a specific configuration of the
developing device 4 in which the two moving mechanisms respectively
assigned to the coating device and developing roller 402 are
constructed substantially integrally with each other. As shown, a
support member supporting the shaft 402a of the developing roller
402 includes a bearing portion formed with a slot 490 elongate in
the direction normal to the drum 1. A spring or biasing means 491
is received in the slot 490 for constantly biasing the developing
roller 402 against the drum 1. In FIG. 31, when the moving
mechanism assigned to the coating device releases the developing
roller 402 from the drum 1, the developing roller 402 is moved
toward the drum i along the slot 490 under the action of the spring
491. As a result, as shown in FIG. 32, the developing roller 402 is
automatically released from the intermediate roller 405.
[0175] In the developing device in which the intermediate roller
405 is rotated in the opposite direction to the developing roller
402, a heavier drive torque is necessary than in the developing
device in which the roller 405 is rotated in the same direction as
the roller 402. In addition, the drive torque is generally heavy at
the initial stage of drive and decreases during continuous, drive.
Considering such a drive torque, the illustrative embodiment causes
the developing roller 402 and intermediate roller 405 to rotate
while being spaced from each other at the initial stage of drive
and then brings them into contact with each other. This
successfully reduces the initial drive torque. It is preferably to
cause the developing roller 402 and intermediate roller 405 to stop
rotating after releasing them from each other.
[0176] At stated above, the illustrative embodiment protects the
developing roller 402 from deformation and protects the thin
developer layer from irregularity and stripes ascribable to
impurities introduced in the developing device 4, thereby insuring
stable, high image quality. Further, the illustrative embodiment
insures the durability of the developing roller 402 and
intermediate roller 405 by so preventing the entry of impurities,
maintaining the quality of the developing device 4 itself
stable.
[0177] A continuous image forming test conducted with the
illustrative embodiment proved that even after the output 20,000
prints, the degradation of image quality ascribable to the
scratches of the developing roller 402 derived from mechanical
stress was not observed. Moreover, when the surface of the drum 1
was formed of a-Si, the drum 1 achieved higher mechanical strength
and therefore longer life than when the surface was formed of
OPC.
Third Embodiment
[0178] Referring to FIG. 33, another alternative embodiment of the
present invention also implemented as an electrophotographic copier
will be described hereinafter. As shown, as for basic construction,
the illustrative embodiment is generally identical with the
embodiment shown in FIG. 4 or 20. The following description will
concentrate on features unique to the illustrative embodiment in
order to avoid redundancy.
[0179] This embodiment is also capable of obviating the degradation
of image quality ascribable to the scratches of the intermediate
roller 502 or those of the developing roller 402 derived from
mechanical stress even after 20,000 prints are output, as also
determined by a continuous image forming test. However, when the
continuous image forming test was extended, the surface of the
intermediate roller 405 was scratched before the developing roller
402 and lowered image quality. Stated another way, the life of the
intermediate roller 405 was shorter than the life of the developing
roller 402, making the coating of the developing liquid on the
roller 402 unstable before the life of the roller 402 ended. The
illustrative embodiment solves this problem with any one of
specific examples to be described hereinafter.
EXAMPLE 1
[0180] To further enhance the durability of the structural elements
while preserving high image quality, Example 1 executed continuous
image forming tests by paying attention to the surface roughness of
the intermediate roller 405. FIG. 34 shows the results of the
continuous image forming tests. As. shown, when the intermediate
roller 405 has its surface roughness increased to a certain degree,
the scratches of the roller 405 sharply decrease, i.e., the life of
the roller 405 is extended. In light of this, in Example 1, the
intermediate roller 405 is provided with surface roughness Rz of 3
.mu.m or above in terms of ten-point mean roughness.
[0181] FIG. 35 shows a relation between the surface roughness of
the intermediate roller 405 and the transfer ratio of the
developing liquid from the roller 405 to the developing roller 402,
as determined by experiments. As shown, the transfer ratio
decreases with an increase in the surface roughness of the
intermediate roller 405. This means that the amount of the
developing liquid passed through the nip between the intermediate
roller 405 and the developing roller 402 increases with an increase
in surface roughness. Example 1 presumably protects the
intermediate roller 405 and developing roller 402 sliding on each
other from scratches because of such an increase in the amount of
the developer passed through the above nip.
EXAMPLE 2
[0182] The toner content of the developing liquid left on the
developing roller 402 after development differs from the image
portion to the non-image portion of the drum. Therefore, if the
developing liquid is again coated on the developing roller 402 for
the next development over the residual developing liquid, then the
ghost of the previous image pattern remains. It is therefore
necessary to remove the developing liquid used for the previous
development from the developing roller 402 before again coating the
roller 402.
[0183] It is a common practice with the developing device to remove
the residual developing liquid from the developing roller 402 by
use of a blade held in contact with the surface of the roller 402.
The blade, however, is apt to scratch the developing roller
402.
[0184] In light of the above, in Example 2, the cleaning blade 411
is held in contact with the intermediate roller 405. More
specifically, the toner left on the developing roller 402 is
removed by the intermediate roller 405, and then the cleaning blade
411 removes the developing liquid collected by the intermediate
roller 405. The developing liquid removed by the cleaning blade 411
is not directly used for development, but is conveyed to the
density adjusting section stated previously. After the density
adjusting section has adjusted the toner content of the collected
developing liquid, the liquid is returned to the coating device.
With this configuration, it is possible to reduce the mechanical
load on the surface of the developing roller 402 for thereby
extending the life of the roller 402 while preserving high image
quality.
[0185] In the case where the intermediate roller 405 removes the
residual developing liquid from the developing roller 402, as
stated above, the surface roughness of the roller 405 has influence
on the cleaning of the roller 402. The transfer ratio shown in FIG.
35 may be translated into a cleaning ratio. Experiments showed that
when the transfer ratio was less than 70%, the ghost of the
previous image pattern appeared. In Example 2, the surface
roughness Rz of the intermediate roller 405 is selected to be 15
.mu.m or below in terms of ten-point mean roughness to thereby
implement the transfer ratio of 70% or above. With this
configuration, the intermediate roller 405 can remove the residual
developing liquid from the developing roller 402 and reduce the
mechanical load on the roller 402 while obviating a ghost.
EXAMPLE 3
[0186] Generally, as shown in FIG. 36, the cleaning blade 411 is
held in contact with the intermediate roller 405 in either one of
two different positions with respect to an angle .theta.. More
specifically the cleaning blade 411 contacts the intermediate
roller 405 in the counter direction when the angle .theta. is
smaller than 90.degree. or contacts it in the trailing direction
when the angle .theta. is greater than or equal to 90.degree.. As
for the trailing position, the residual developing liquid removed
by the cleaning blade 411 turns round in the lengthwise direction
of the blade 411 in the wedge-like space between the blade 411 and
the roller 405 and therefore deposits more on the end of the roller
411. Part of the developing liquid deposited on the end of the
roller 405 cannot be collected, but is returned to the coating
device without having its toner content adjusted. The trailing
position therefore makes it difficult to control the toner content
of the developing liquid.
[0187] By contrast, if the cleaning blade 411 is held in the
counter direction and made thin, it is possible to reduce the
turn-round of the developing liquid in the lengthwise direction of
the blade 411. In Example 3, therefore, the cleaning blade 411 is
held in contact with the intermediate roller 405 in the counter
direction and implemented as a rubber blade as thin as 1 mm. The
rubber blade is adhered to a 0.2 .mu.mm thick sheet of metal If the
cleaning blade 411 should be pressed against the intermediate
roller 405 with sufficient pressure, then the blade 411 must be as
thick as about 3 mm.
[0188] As stated above, the cleaning blade 411, contacting the
intermediate roller 405 in the counter direction, reduces the
residual developing roller that it could not collect in the
trailing direction and thereby promote adequate control over the
toner content of the developing liquid.
EXAMPLE 4
[0189] Impurities introduced in the developing liquid are apt to
produce stripes in the developer layer formed on the developing
roller 402, thereby rendering development defective. Moreover, if
such impurities are higher in hardness than the surface of the
developing roller 402, then the impurities are likely to scratch
the surface of the roller 402 and reduce the life of the roller
402. While the second embodiment stated earlier arranges the
coating device and developing roller 402 in the substantially
hermetically closed container in order to solve the above problem,
it is difficult to fully hermetically confine the developing device
4 or the coating device, i.e., to fully obviate the introduction of
impurities in the developing liquid.
[0190] Experiments were conducted to see how image quality and the
life of a roller were effected by scratches formed on the surface
of the roller by impurities. The experiments showed that dust and
other impurities accumulated at the edge of a cleaning blade, which
contacted the roller surface, and caused scratches to extend from
such a position over the circumference of the roller. More
specifically, when the coating device continuously coated the
developing liquid on the developing roller 402, impurities E
gathered between the surface of the intermediate roller 405 and the
cleaning blade 411 and caused scratches to extend from the
impurities over the circumference of the roller 405. Such
impurities E should preferably be removed because it is difficult
to fully obviate the entry of impurities in the developing device
4, as stated earlier.
[0191] To remove the impurities E, the cleaning blade 411 maybe
released from the intermediate roller 405. However, if the
intermediate roller 405 is rotated after the release of the
cleaning blade 411, then the roller 405 conveys the impurities to
the nip between the roller 405 and the coating roller 404, causing
the impurities E to again appear at the coating position.
[0192] In light of the above, Example 4 includes rotation switching
means for switching the direction of rotation of the intermediate
roller 405. The rotation switching means may be implemented as a
driveline using a reversible stepping motor or a reversible DC
motor. When the rotation switching means causes the intermediate
roller 405 to rotate in the direction opposite to the usual
direction assigned to coating, the impurities E are pulled out of
the nip (cleaning position) between the cleaning blade 411 and the
roller 405, as shown in FIG. 37B. At this time, the cleaning blade
411 contacts the intermediate roller, which is rotating in the
reverse direction, in the trailing direction, allowing the
impurities E to easily move away from the cleaning position. This
successfully protects the surface of the intermediate roller 405
from scratches ascribable to the impurities E. Subsequently, as
shown in FIG. 37C, the rotation switching means again causes the
intermediate roller 405 to rotate in the usual direction, so that
the cleaning blade 411 can remove the impurities E from the
intermediate roller 405.
EXAMPLE 5
[0193] In Example 5, the distance over which the surface of the
developing roller 405 moves in the reverse direction, as stated
above, is selected to be smaller than the distance between the nip
between the roller 405 and the cleaning blade 411 and the nip
between the roller 405 and the developing roller 402. Stated
another way, the angular movement of the intermediate roller 405 in
the reverse direction is selected such that the cleaning position
at the beginning of the reverse rotation does not reach the nip
between the rollers 405 and 402, see FIG. 37B. In this condition,
the impurities pulled out of the cleaning position are prevented
from reaching the nip between the rollers 405 and 402 and
depositing on the roller 402 when the roller 405 is rotated in the
reverse direction.
EXAMPLE 6
[0194] Even when the cleaning roller 411 removes the impurities
when the intermediate roller 405 is rotated in the usual direction
as in Example 5, the impurities E are apt to again stay at the
cleaning position. More specifically, it is, in practice,
impossible to fully remove the developing liquid from the
intermediate roller 405 with the cleaning blade 411. As a result,
some developing liquid presumably moves past the cleaning position
and causes the impurities E to stay at the cleaning position.
[0195] Generally, when a liquid passes through a nip between two
objects pressed against each other, the passage becomes more
difficult as the moving speed of the liquid increases. Taking this
fact into account, Example 6 makes the rotation speed of the
intermediate roller 405 lower when the impurities E pulled out of
the cleaning position are again brought to the cleaning position
than the usual rotation speed assigned to coating, see FIG. 37C.
The lower speed of the intermediate roller 405 reduces the amount
of the developing liquid to pass through the cleaning position for
thereby allowing a minimum of impurities to stay at the cleaning
position.
[0196] To control the rotation speed of the intermediate roller
405, use may be made of a speed control motor or a stepping motor.
In Example 6, a stepping motor is used in order to control the
amount of rotation of the intermediate roller 405 on the basis of
the number of steps of the motor. More specifically, a stepping
motor is selectively rotatable at an initial speed or a steady
operation speed and can be accelerated for a preselected period of
time. As shown in FIG. 38, in Example 6, the initial speed and the
duration of acceleration are controlled such that the rotation
speed of the intermediate roller 405 is lowered when the impurities
E are again brought to the cleaning position. This makes it
needless to set two different rotation speeds of the intermediate
roller 405 that would make control sophisticated.
EXAMPLE 7
[0197] The impurities removed from the intermediate roller 405 at
the cleaning position are conveyed to the density or toner content
adjusting section together with the developing liquid. A filter,
not shown, is disposed on the path connecting the cleaning position
and the toner content adjusting section and filters out the
impurities E. The filter, however, needs periodic replacement
because it is stopped up as the time elapses. It is therefore not
efficient to convey the impurities E pulled out of the cleaning
position to the filter via the above path.
[0198] In light of the above, as shown in FIGS. 39A and 39B,
Example 7 includes a removing member 700 for removing the
impurities E deposited on the intermediate roller 405. As shown,
the removing member 700 is selectively movable into or out of
contact with the intermediate roller 405 at a position between the
developing roller 402 and cleaning blade 411 and removes the
impurities E pulled out of the cleaning position. While the
removing member 700 may be implemented as, e.g., a roller, it
should preferably be implemented as a webbing when consideration is
given to the removal of the impurities E from the intermediate
roller 405, as illustrated. The webbing is implemented by unwoven
cloth of cotton that is highly hygroscopic and releases a minimum
of substances.
[0199] As shown in FIG. 39A, during usual coating operation, the
removing member 700 is spaced from the intermediate roller 405. As
shown in FIG. 39B, at the time when the intermediate roller 405 is
rotated in the reverse direction to allow the impurities to be
removed, the removing member 700 is brought into contact with the
roller 405 for thereby removing the impurities E. The webbing,
constituting the removing member 700, can be taken up to replace
its portion smeared with the impurities E with a fresh portion,
preventing the removed impurities E from being returned to the
intermediate roller 405.
[0200] As stated above Examples 4 through 7 can periodically remove
the impurities that would otherwise scratch the surface of the
intermediate roller 405, would lower image quality, and would
reduce the life of structural parts. The removal of the impurities
E is automatically effected after a preselected amount of
developing liquid has been coated on the intermediate roller 405,
e.g., every time 50,000 prints are output. Alternatively the
removal of the impurities may be automatically effected every time
a power switch, not shown, is turned on or may even be effected by
the user any time in accordance with an operation manual.
[0201] The smooth layer 405c of the intermediate roller 405 is
formed by coating a coating agent containing fluorocarbon resin on
the elastic layer 405b to thickness of 10 .mu.m to 50 .mu.m; the
elastic layer 405b is formed of urethane resin and has hardness of
40.degree. in terms of JIS-A scale. While the smooth layer 405c may
be implemented by, e.g., a PFA tube, as stated earlier, the PFA
tube determines the surface roughness of the intermediate roller
405 alone. It is difficult to control the surface roughness of a
tube and therefore the surface roughness of the intermediate roller
405, To solve this problem, in the illustrative embodiment, the
smooth layer 405c is formed on the elastic layer 405b by coating. A
coating method, which may be dipping, spraying or the like, is
dependent on the material of the intermediate roller 405 and
coating material.
[0202] As for the method stated above, the surface roughness of the
intermediate roller 405 may be controlled either by (1) processing
the surface layer of the elastic layer and (2) selecting an
adequate coating method and an adequate material for the coating
layer. The illustrative embodiment uses the above scheme (1).
Generally, a rubber roller is produced by forming a rubber layer or
elastic layer on a metallic core and then grinding the surface of
the rubber layer. In the illustrative embodiment, the surface
roughness of the intermediate roller 405 is controlled by grinding.
For a given coating condition, surface roughness after coating
varies substantially in proportion to surface roughness before
coating and is 0.5 time to 0.8 time greater than the latter in
terms of Rz value although dependent on conditions, as shown in
FIG. 40. Therefore, if the elastic layer 405b is ground with the
above variation being taken into account, then the surface
roughness after coating can be controlled.
[0203] The other scheme (2) is difficult to practice because
coating itself originally tends to reduce surface roughness.
Although grains for controlling surface roughness may be introduced
in a coating material, such grains bring about another problem that
surface roughness becomes irregular due to, e.g., short dispersion.
In this sense, the scheme (1) is advantageous over the scheme (2)
in the aspect of stable control over surface roughness and
preservation of the ability.
[0204] In the developing device 4 in which the developing liquid is
coated on the developing roller 402, the roller, like the
intermediate roller 405 should preferably be provided with surface
roughness Rz of 3 .mu.m or above. Such surface roughness of the
developing roller 402 is considered to obviate scratches ascribable
to mechanical loads, e.g., sliding contact of the roller 402 with
the intermediate roller 405. However, a problem with the developing
roller 402 is that if its surface roughness is excessively great,
then irregularity in surface roughness appears in, e.g., a halftone
image. Experiments were conducted by varying the surface roughness
of the developing roller 402 and showed that surface roughness Rz
of 5 .mu.m or below obviated the above problem. The illustrative
embodiment insures high-quality images with a roller covered with a
PFA tube and having surface roughness Rz of 2 .mu.m to 3 .mu.m.
[0205] A continuous image forming test conducted with the
developing device 4 of the illustrative embodiment showed that even
when 400,000 prints were output, image quality was free from
degradation ascribable to the scratches of the intermediate roller
405 brought about by mechanical stress. Further, the drum 1
achieves greater mechanical strength and therefore a longer life
when implemented by a-Si than when implemented by OPC.
[0206] Various modifications will become possible for those skilled
in the, art after receiving the teachings of the present disclosure
without departing from the scope thereof.
* * * * *